1. Field of the Invention
The present invention relates to an electron beam exposure apparatus, and more particularly, to improvement of calibration precision of position and beam size in the electron beam exposure apparatus.
2. Description of Related Art
Conventionally, in order to pattern a resist layer on a semiconductor wafer, an optical exposure apparatus has been used widely. However, as a high density of integration is required, a pattern becomes thin. Recently, a drawing precision of 0.05 .mu.m or below is required for the exposure apparatus. Therefore, an electron beam exposure apparatus becomes interested. In an electron beam exposure apparatus, electron beam emitted from an electron gun is contracted thinly by an optical system and scanned by a deflecting unit to expose a resist layer. The electron beam exposure apparatus has an advantage in that a fine pattern can be formed, compared to the optical exposure apparatus such as a stepper. Also, the electron beam exposure apparatus has an advantage in that a mask is not used and the fine pattern is drawn on the resist layer directly by scanning the electron beam. However, the optical system of the electron beam exposure apparatus has an aberration inherent to the apparatus, that is to say, the aberration determined with mechanical precision when the apparatus is produced. Accordingly, an actual position of a deflected electron beam on a semiconductor wafer is displaced from a target position on the wafer. The displacement is the deflection distortion. For this reason, therefore, the defection of electron beam must be controlled with high precision and the optical system of the electron beam exposure apparatus must be calibrated for correction for deflection distortion before the resist pattern is actually exposed by the electron beam. Similarly, there is a difference between a desired beam size and an actual beam size. Therefore, the beam size needs to be also corrected based on the difference in beam size.
In a conventional electron beam exposure apparatus, a calibration reference marker provided on a stage on which a semiconductor wafer is mounted is generally formed a bulk of metal. As shown in FIG. 1A, the calibration reference marker 114 has two sections, i.e., a projection section and a base section. In the calibration, when an electron beam 172 is irradiated on the calibration reference marker 114 and scanned in a direction shown by an arrow 170, electrons 174 reflected from the marker are detected by a detector to produce a reflected electron signal and the calibration is performed based on the reflected electron signal. The waveform when the reflected electron signal is differentiated by the inventor is shown in FIG. 1B. In this example, since the reference marker is formed of the bulk of metal so that electrons reflected from the base section provides background noise, an S/N ratio of the reflected electron signal from the projection section to the background noise is low so that detection precision of the deflection distortion and beam size are also low.
The provision of a marker for detecting an exposure position which is not a calibration reference marker is disclosed in, for example, Japanese Laid Open Patent Disclosure (JP-A-Sho63-278350). As shown in FIG. 2A, the marker 150 made of heavy metal such as tungsten (W) or tantalum (Ta) is provided in a semiconductor structure 112 formed of a silicon layer, an aluminum layer or a silicon oxide (Sio.sub.2) layer. In this example, an electron beam is irradiated on the marker on the semiconductor structure and reflected electrons are detected by a detector. FIG. 2B shows the waveform when the detected reflected electron signal is differentiated by the inventor. In this case, since the number of electrons reflected from the semiconductor structure 112 is less than that of electrons reflected from the marker 150, background noise is reduced so that the S/N ratio can be improved. However, the number of electrons reflected from the semiconductor structure 112 is sufficiently great and more than about 20% to 30% electrons in the incident electron beam are reflected to provide the background noise. As a result, the S/N ratio is as many as 2 to 3. This value corresponds to the detection precision of 0.07 .mu.m. As described above, the drawing precision of 0.05 .mu.m or below is required recently and the value obtained from the device shown in FIG. 2A cannot satisfy the required drawing precision. Therefore, there is the need for an electron beam exposure apparatus having the drawing precision of 0.05 .mu.m or below.